1. Field of the Invention
The present invention relates to a quartz crystal unit having a high vibration frequency of 100 MHz or higher, and more particularly to a crystal unit having good aging characteristics and oscillating characteristics and a holding structure of such a crystal unit.
2. Description of the Related Art
Crystal units having a quartz crystal blank housed in a casing are incorporated as a frequency control device, particularly a reference source of a communication frequency, in oscillators. In recent years, with the advent of optical communication systems, there have been demands for crystal units having higher vibration frequencies. To meet such demands, a crystal unit has been developed which has a recess defined in a resonating region of a crystal blank to reduce the thickness of the crystal blank in the recess for increasing the resonant frequency, and holds the resonating region with a relatively thick portion around the recess to provide desired mechanical strength.
FIG. 1 shows a conventional crystal unit of such a design in exploded perspective. As shown in FIG. 1, the conventional crystal unit comprises casing 5 made of laminated ceramics and having a recess defined therein, and crystal blank 2 accommodated in the recess in casing 5. Casing 5 is in the form of a substantially rectangular parallelepiped, and crystal blank 2 is of a substantially rectangular shape. A step is formed on one of side surfaces of the recess in casing 5. A pair of connecting terminals 7 is disposed on the upper surface of the step at its opposite ends for electrically connecting to crystal blank 2. A pair of mounting terminals (not shown) is disposed on the outer surface of casing 5 and used to surface-mount the crystal unit on a wiring board. The mounting terminals are electrically connected to connecting terminals 7 through via holes or the like that are defined in casing 5.
Structural details of crystal blank 2 are shown in FIGS. 2A and 2B. Crystal blank 2 typically comprises an AT-cut quartz crystal blank. The AT-cut quartz crystal blank has its resonant frequency determined depending on the thickness thereof. The resonant frequency of the AT-cut quartz crystal blank is higher as the thickness thereof is smaller. To enable crystal blank 2 to have a resonant frequency in excess of 100 MHz, hole portion 1 is defined centrally in one principal surface of crystal blank 2, making crystal blank 2 thinner at the bottom of hole portion 1 than at a portion around hole portion 1 with the thinner region serving as a vibrating region. In the vibrating region, excitation electrodes 3 are disposed respectively on the principal surfaces of crystal blank 2. Extension electrodes 4 extend respectively from excitation electrodes 3 toward respective opposite areas of a shorter side of crystal blank 2. Extension electrodes 4 are associated respectively with connecting terminals 7 on the step of casing 5. Extension electrode 4 which is disposed on the upper surface of crystal blank 2 as shown has its tip end folded back over the lower surface of crystal blank 2 as shown across the shorter side of crystal blank 2. The both ends of the shorter side of crystal blank 2 is fixed to connecting terminals 7 by electrically conductive adhesive 6, thus holding crystal blank 2 horizontally in the recess in casing 5 and electrically connecting extension electrodes 4 to connecting terminals 7. Therefore, the mounting terminals disposed on the outer surface of casing 5 are electrically connected to excitation electrodes 3 of crystal blank 2.
After crystal blank 2 is fixed to the step in the recess in casing 5, a cover (not shown) is placed on casing 5 to seal the opening of the recess, thus hermetically sealing crystal blank 2 in casing 5.
The crystal unit of the above structure has suffered the following problems because of crystal blank 2 being fixed in position by electrically conductive adhesive 6: Electrically conductive adhesive 6 comprises a polymer resin such as silicone or epoxy resin mixed with metal particles. When electrically conductive adhesive 6 is hardened with heat, it bonds crystal blank 2 to casing 5. When electrically conductive adhesive 6 is thermoset, it emits an organic gas that is attached to crystal blank 2. An organic gas component that remains unremoved by cleaning or the like will subsequently be released and attached again to the vibrating region of crystal blank 2. When the assembly is exposed to a high temperature after crystal blank 2 is hermetically sealed in casing 5, electrically conductive adhesive 6 also emits an organic gas that will subsequently be attached to the vibrating region of crystal blank 2. If the organic gas component is attached to the vibrating region of crystal blank 2, the vibration frequency of crystal blank 2 or the like changes due to the mass addition effect or the like of the organic gas component, impairing the aging characteristics of the crystal unit. The higher the vibration frequency of crystal blank 2, the smaller the thickness of the vibrating region. Thus, if the vibration frequency of crystal blank 2 is higher, then the organic gas component that is attached to the vibrating region of crystal blank 2 is more detrimental to the aging characteristics of the crystal unit.
One solution would be to use a eutectic alloy having a low melting point, e.g., an inorganic material of AuSn (gold-tin) alloy, as a joining material for fixing crystal blank 2 to casing 5, instead of electrically conductive adhesive 6. However, since the joint made by a eutectic alloy has a high bonding strength, crystal blank 2 is strained due to the difference between the coefficients of thermal expansion of crystal blank 2 and casing 5. FIGS. 3A and 3B show the manner in which crystal blank 2 is strained due to the difference between the coefficients of thermal expansion of crystal blank 2 and casing 5. As shown in FIGS. 3A and 3B, crystal blank 2 is bent under stresses applied between two outer peripheral points on crystal blank 2. The strain is propagated to the vibrating region of crystal blank 2, impairing the vibrating characteristics of crystal blank 2, in particular, frequency vs. temperature characteristics that will be represented by a cubic function curve if crystal blank 2 comprises an AT-cut quartz crystal blank.
If crystal blank 2 is held at its opposite ends, then it will be strained to a greater extent because the vibrating region is positioned between the opposite ends than if crystal blank 2 were held at opposite sides at one end thereof. If the vibration frequency of crystal blank 2 is higher, then since the thickness of the vibrating region is smaller, crystal blank 2 will be strained more greatly. Though crystal blank 2 is also strained if it is fixed in position by electrically conductive adhesive 6, it is trained to a much larger extent if the eutectic alloy is used as the joining material.